(1) Field of the Invention
The present invention relates to a multilayer capacitor housing a capacitor element in an outer case, the capacitor element being comprised of a plurality of metal foils alternately stacked with electrically insulating separators interposed therebetween, the metal foils partly comprising connecting portions, and connecting the above-mentioned connecting portions respectively to a positive electrode external terminal and a negative electrode external terminal, and a method for manufacturing the multilayer capacitor.
(2) Related Art Statement
A multilayer capacitor using the electrolytic capacitor element formed by stacking or coiling the anode foil, where a metal foil made of valve metal such as aluminum is processed with surface enlargement for increasing its surface area and an oxide layer, which is a dielectric layer, is formed by anode oxidation, and the cathode foil, where a metal foil made of the above-mentioned valve metal such as aluminum is processed only with the above-mentioned surface enlargement, with an electrolytic paper interposed therebetween as the above-mentioned electrically insulating separator, the electrolytic paper being impregnated with an electrolyte, or a multilayer capacitor using the electric double layer capacitor element formed by stacking or coiling the paired polarizable electrode foils of an anode and cathode made by forming activated charcoal layers on both sides of a metal foil made of valve metal such as aluminum, with an electrolytic paper interposed therebetween as the electrically insulating separator, the electrolytic paper being impregnated with an electrolyte, have been conventionally known as the multilayer capacitor. The multilayer capacitors using such capacitor elements are made to be multilayer electrolytic capacitors by bundling together and stacking successively a plurality of connecting portions for each electrode, the connecting portion being protrudingly formed in the circumference of each electrode foil, connecting each bundled and stacked connecting portion to form a capacitor element, housing said capacitor element in an outer case, and sealing an aperture with a sealing material (Patent Document 1).
[Patent Document 1]
Patent Application Unexamined Publication No.H04-154106
For the multilayer electrolytic capacitors using the capacitor elements, it is necessary to electromechanically connect and combine each connecting portion protrudingly formed in the circumference of each metal foil. Stitch welding or ultrasonic welding is mainly used as the above method.
The stitch used for such stitch welding method must be increased in size in proportion as the number of stacked layers increases and the thickness of the stacked layers becomes thick. Thus, this method is not suitable for the case of connecting a plurality of connecting portions.
In addition, for the ultrasonic welding, etched layers for surface enlargement are formed on the surfaces of the anode and cathode foils stacked or coiled, or the oxide layer formed by a formation process is formed on the anode foil as stated above. Because carrying out masking treatment or the like so as not to form the etched layers or oxide layers only on the above-mentioned connecting portions complicates its process, the etched layers or oxide layers are formed on those connecting portions as well for the purpose of simplifying the process. In order to weld each aluminum that is the base metal of each connecting portion over the etched layers and oxide layers formed on the surfaces of both connecting portions to be connected, it becomes necessary to give a bigger ultrasonic vibration. Thus, if a big ultrasonic vibration is given to connect all the connecting portions in a good condition including the bottom of the successively stacked connecting portions, there may occur a problem that the top of the successively stacked connecting portions is ruptured due to the applied big ultrasonic vibration. Alternatively, if the applied ultrasonic vibration is controlled so as not to generate the rupture, the bottom of the successively stacked connecting portions can not be connected in a good condition.
Also, when metal foils comprising polarizable electrode layers made primarily of activated charcoal or carbon on their surfaces instead of the above-mentioned oxide layers formed by the formation process are connected, the same problem as above may occur.
Also, this ultrasonic welding may be carried out on the above-mentioned connecting portions where the etched layers or oxide layers are not formed by carrying out the masking treatment or the like. In this case, although the connecting portions are connected in a better condition compared with the ones comprising the etched layers and oxide layers, a boundary layer remains between the metal foils, each of which is the connecting portion, and said metal foils are easily broken away from their interface. Thus, the connection strength is not enough.
Accordingly, the present invention focuses on the above described problems, and intends to provide a multilayer capacitor capable of connecting the above-mentioned connecting portions in a good condition regardless of the etched layers, oxide layers, and polarizable electrode layers.